Built-in _stats Function
The built-in _stats reduce function produces statistical calculations for the input data.
As with the _sum function, the corresponding value in the emit call should be a number.
The generated statistics include the sum, count, minimum ( min ), maximum ( max ) and sum squared ( sumsqr ) of the input rows.
For example, using the sales data, a slightly truncated output at group level one shows the same fields in the output value for each of the reduced output rows.
{
"rows" : [
{
"value" : {
"count" : 3,
"min" : 7000,
"sumsqr" : 699000000,
"max" : 19000,
"sum" : 43000
},
"key" : [
"Adam"
]
},
{
"value" : {
"count" : 3,
"min" : 5000,
"sumsqr" : 594000000,
"max" : 20000,
"sum" : 38000
},
"key" : [
"James"
]
},
{
"value" : {
"count" : 3,
"min" : 3000,
"sumsqr" : 542000000,
"max" : 22000,
"sum" : 32000
},
"key" : [
"John"
]
}
]
}